Signalling via Intracellular Receptors

Question 1

What is NO made from?

Answer 1

L-arginine

Question 2

What is the intermediate molecule of making NO from L-arginine?

Answer 2

N-hydroxyarginine

Question 3

What is the byproduct of making NO from L-arginine?

Answer 3

L-citralline

Question 4

What is the enzyme that catalyses L-arginine to NO?

Answer 4

Nitric oxide synthase (NOS)

Question 5

Describe how NOS catalyses the synthesis of NO?

Answer 5

Through two serial monooxygenase reactions analogous to those of the NADPH-dependent cytochrome PH450 oxidoreductase system

Question 6

What is cNOS?

Answer 6

Constitutive NOS. Its enzymatic activity is induced by calcium/calmodulin. It is transiently active

Question 7

What are the two forms of cNOS?

Answer 7

eNOS (endothelial - bound to cell membrane) and nNOS (neural - cytosolic)

Question 8

Where is eNOS expressed?

Answer 8

Endothelium, cardiac myocytes, renal mesangial cells, osteoblasts, platelets)

Question 9

Where is nNOS expressed?

Answer 9

CNS, NANC neurons, ENS, retina

Question 10

What is iNOS?

Answer 10

Inducible NOS, transcription is inducible and it is always active. Pathological stimuli activate their transcription. Main role is in the immune system.

Question 11

Where is iNOS expressed?

Answer 11

Macrophages

Question 12

How is NO induced?

Answer 12

1. ACh released from synapse and it activates NO synthase (cNOS) in endothelial cells
2. Activated NOS generates NO from arginine
3. NO binds to guanylyl cyclase, converting GTP to cGMP
4. cGMP activates cGMP-dependent protein kinase (PKG), which converts myosin light chain phosphatase into myosin light chain which causes muscle relaxation

Question 13

What affect does NO have on vasculature?

Answer 13

Autonomic nerves release ACh which acts on endothelial cells
Endothelial cells release NO which causes surrounding smooth muscle to relax
NO also inhibits platelet aggregation and vascular smooth muscle proliferation

Question 14

What is the effect of nNOS on the brain?

Answer 14

nNOS is tethered close to NMDA-type glutamate receptors in the CNS so it can respond to the Ca2+ increase near open channels
Glutamate produced by the presynaptic terminal activates glutamate receptors (NMDA) which stimulates the NO pathway
NO travels to the presynaptic terminal and increases the production of glutamate, creating the state of long term potentiation

Question 15

What is the role of NO in the immune system?

Answer 15

Kills bacteria and parasites and induces programmed cell death

Question 16

What is the role of iNOS in inflammatory cells?

Answer 16

It uses NO as a cytostatic and cytotoxic agent. It produces higher numbers of NO

Question 17

What happens if NO is overproduced by iNOS?

Answer 17

Can trigger inflammatory diseases in the lungs and intestines (e.g. Crohn’s disease, rheumatoid arthritis, asthma)